132 LIFE ON THE EARTH. 



physical and social, which augment or lessen its 

 amount on any given area of the globe. Take the 

 single case of Ireland. The potato, naturalised as a 

 food of the country, raised its population to the inordi- 

 nate amount of eight millions. The potato disease of 

 1845 and the following years (the work of one of the 

 lowest forms of parasitic life) took off nearly a quarter 

 of this number by famine or emigration. In every 

 ^rade below man in the animal kingdom life is simi- 

 larly subjected to the conditions which press upon it 

 from without, either such as are purely physical or 

 the conflicts of different forms of animal life with one 

 another. This is in great measure a new branch of 

 knowledge, and it has already been rendered a prolific 

 one. We see how not only the quantity but also the 

 organisation and habitudes of life depend on local and 

 ever-varying circumstances, and how far man is con- 

 cerned in fixing or changing these. Much, however, 

 yet remains to be done, as well in correcting errors of 

 older date as in determining the laws which pertain to 

 this part of the great problem of life on the earth. 



In close connexion with this subject, and its most 

 important correlation, comes the question regarding 

 the relations of man to the other forms of animal crea- 

 tion peopling the earth. Surrounded on every side by 

 living beings, consuming them, consciously or uncon- 

 sciously, as food, and even inhaling them with every 

 breath, this question must ever be one of deep interest 

 to human thought and well worthy all that can be 

 done towards its solution. 



Bayle says, and justly, ' Les actions des betes sont 



